Get on top of the coffin again and float along with the current to the intersection near the metal gate where the water is flowing out of this area. (You can grapple the wall sconce and pull to go faster if you like.)
Turn so that gate is behind Lara and you'll see a silver reward (4/5) in a raised alcove at the end of the passageway straight ahead. Grapple the sconce beside the alcove and pull the coffin back along the passage toward the alcove.
When you've pulled the coffin as close as possible, release the cable and jump up onto the ledge. Grab the reward and quickly jump back down onto the coffin.
If the coffin floats away while you're getting the reward, swim over to it, climb on top, grapple the sconce near the alcove where the reward was, and pull to drag the coffin back that way.
Turn so the alcove where you got the reward is behind Lara. Grapple the sconce next to the gate ahead, where the water flows out of this area, and pull.
Position the coffin close enough to the gate so you can jump on top of it. Then jump into the water on the other side and let the current carry Lara away.
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